Brown crystal aggrergtaes of the very rare Fe arsenate suphate mineral sarmientite from one of the very few localities known worldwide: Winnemucca, Humboldt County, Nevada, United States of America. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Sarmientite
Sarmientite is an IMA-recognized mineral species in the arsenate class, with the chemical formula Fe³⁺₂(AsO₄)(SO₄)(OH)·5H₂O.
It crystallizes in the monoclinic crystal system (space group P2_1/c).
Key properties: specific gravity 2.58; optically biaxial.
IMA symbol Smi, grandfathered status — known before the IMA’s founding, named in 1941.
A rare mineral found in oxidized pyritic veins in diabase.
It is found associated with Fibroferrite, copiapite, botryogen, szomolnokite, zincian melanterite, slavı́kite, epsomite, gypsum.
The mineral was named to honor Domingo Faustino Sarmiento (1811-1888), former President of Argentina.